1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an electrophotographic image forming method. In addition, the present invention also relates to a developing device and a process cartridge for the image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes.
(1) An electrostatic image is formed on an image bearing member such as photoreceptors (electrostatic latent image forming process);
(2) The electrostatic image is developed with a developer including a toner to form a toner image on the image bearing member (developing process);
(3) The toner image is transferred to a receiving material optionally via an intermediate transfer medium (transfer process); and
(4) The toner image is fixed on the receiving material (fixing process).
The developers are broadly classified to two component developers which include a toner and a carrier, and one-component developers (such as one-component magnetic developers and one-component non-magnetic developers) which include no carrier and which consist essentially of a toner.
In two-component developing methods, toner particles tend to be fixedly adhered to the surface of carrier particles after long repeated use. In this case, the properties (such as charging ability) of the carrier particles deteriorate. In addition, since only the toner is used for developing electrostatic images, the toner concentration in the developer decreases with time, and therefore a device for controlling the toner concentration is necessary. Therefore, image forming apparatus using a two-component developing method become large in size.
In contrast, one-component developing methods do not have such drawbacks and therefore it is possible to miniaturize image forming apparatus. In addition, one-component developing methods have another advantage such that the image forming apparatus using the methods can be used under various environmental conditions (from low temperature/low humidity conditions to high temperature/high humidity conditions). Therefore, one-component developing methods are mainly used now.
One-component developing methods are classified into one-component magnetic developing methods using a magnetic toner and one-component non-magnetic developing methods using a non-magnetic toner.
In one-component magnetic developing methods, a thin layer of a magnetic toner, which includes a magnetic material (such as magnetite), is formed on a developing sleeve including a magnet therein using a thickness controlling member (such as blades). Therefore, recently the magnetic one-component developing methods are widely used for small-sized printers.
In contrast, non-magnetic toners for use in the one-component non-magnetic developing methods have no magnetic force. Therefore, in one-component non-magnetic developing methods, a thin toner layer is formed using a toner supply roller which presses a toner to a developing sleeve. Therefore, the toner layer is electrostatically borne on the developing sleeve. Since non-magnetic toners do not include a magnetic material, which is typically colored, the non-magnetic toners can be preferably used for forming color images. In addition, since a developing sleeve including no magnet therein can be used, the developing device (i.e., the image forming apparatus) has a light weight and a low cost. Therefore, recently such non-magnetic one-component developing methods are widely used for small-sized full color printers.
Developing devices typically include a developing section configured to develop an electrostatic image on an image bearing member, and a toner cartridge configured to supply a toner to the developing section. These developing devices often cause a problem in that a large amount of toner particles in the developing section are returned to the toner cartridge, thereby decreasing the amount of the toner in the developing section, resulting in formation of abnormal images.
In attempting to solve the problem, published unexamined Japanese patent application No. (herein after referred to as JP-A) 2002-162817 discloses a developing device which includes a developing section configured to develop an electrostatic image on a photoreceptor belt and a toner cartridge configured to supply a toner to the developing section through a connection passage and which can be detachably attached to an image forming apparatus. In this regard, a check valve made of an elastic material is provided on the connection passage to prevent the toner in the developing section from returning to the toner cartridge.
Since the toner in the developing section does not return to the toner cartridge, toner particles with a low charge quantity tend to remain in this developing section after long repeated use. Therefore, problems in that images having undesired image qualities are formed and toner particles are scattered around the developing section tend to occur. Particularly, in one-component non-magnetic developing methods toner particles having a relatively small particle diameter and a relatively high charge quantity are mainly used for developing electrostatic images. Namely, toner particles having a relatively large particle diameter and a relatively low charge quantity tend to remain in the developing section, thereby affecting the image qualities of the toner images.
In attempting to impart a proper charge amount to a toner, JP-A 2000-227676 discloses a technique in that a binder resin having a high acid value is used for the toner. However, this technique has a drawback in that the resultant toner has poor stability to withstand environmental conditions. Specifically, the toner has too large an amount of charge quantity under low temperature and low humidity conditions, and has too small an amount of charge quantity under high temperature and high humidity conditions. Therefore, the toner causes problems such as decrease of image density and formation of background fouling.
JP-A 2004-354530 discloses a technique in that a large amount of charge controlling agent is included in a toner. However, it is hard to disperse a large amount of charge controlling agent in a binder resin. If a charge controlling agent is nonuniformly dispersed in a toner, image qualities tend to deteriorate after long repeated use.
JP-A 2003-280256 discloses a technique in that a proper amount of charge is imparted to a toner without using a charge controlling agent. Specifically, the technique is that a particulate inorganic material is adhered to the surface of toner particles. However, it is difficult to perfectly solve the problems mentioned above (i.e., decrease of the charge quantity of the toner in the developing section) cannot be avoided by using this technique when the toner is repeatedly used in a developing device.
Because of these reasons, a need exists for an image forming apparatus which can produce high quality images by imparting a proper amount of charge to a toner in a developing section even after long repeated use without deteriorating the fluidity of the toner.